A position adjustment apparatus of a steering wheel, such as a fore-aft position adjustment apparatus called a telescopic steering apparatus, or an up-down position adjustment apparatus called a tilt-type steering apparatus, is widely used as an apparatus for adjusting the position of a steering wheel according to the physical dimensions and posture of a driver. Furthermore, conventionally, a motor-driven position adjustment apparatus for a steering wheel is also widely used, in which the position is adjusted by an electric motor based on a switching operation. For example, FIG. 35 shows a motor-driven fore-aft position adjustment apparatus for a steering wheel, which is disclosed in Patent Document 1.
In the case of this conventional structure, a steering column 1 is a so-called telescopic steering column whose overall length can be extended and contracted freely by inserting the front part of an inner column 3 provided toward the rear (right side in FIG. 35) into an outer column 2 provided toward the front (left side in FIG. 35). The rear end part (right end in FIG. 35) of a driving rod (push-pull rod, threaded rod) 5 is joined and fixed to a joining plate 4 fixed toward the rear end part of the inner column 3. Moreover, a male thread 6 provided on the front end part (left half of FIG. 35) of the driving rod 5 is screwed into a nut which is provided inside a housing 7 fixed on the outer peripheral surface of the outer column 2, such that it can only rotate freely, and which is rotated in a desired direction by an electric motor, to thereby form a feed screw mechanism. When adjusting the fore-aft position of a steering wheel 8, the inner column 3 is pushed or pulled in the axial direction (left/right direction in FIG. 35) by the feed screw mechanism to extend and contract the steering column 1. A steering shaft 9, on the rear end part of which is fixed the steering wheel 8, is supported inside of the inner column 3 such that it can only rotate freely, and can be extended and contracted freely by a spline engagement part. Accordingly, the fore-aft position of the steering wheel 8 can be adjusted as the steering column 1 is extended and contracted.
The construction and operation of a motor-driven steering wheel fore-aft position adjustment apparatus is as described above. However, in order to protect a driver at the time of a collision accident, regardless of the existence of the above-described driving rod 5, a structure is required whereby the steering wheel 8 can be displaced forward. That is, at the time of a collision accident, following a so-called first collision where a vehicle collides with another vehicle or the like, a so-called secondary collision occurs where the driver's body (mainly chest or head) collides with the steering wheel 8. In order to relieve the impact on the driver's body at the time of this secondary collision, a construction is required in which the steering wheel 8 is displaced forward simultaneously with the secondary collision.
In the case of the construction shown in FIG. 35, since the steering column 1 and the steering shaft 9 are constructed such that they can both be extended and contracted freely, it is possible to form a structure directly in which the steering wheel 8 is displaced forward simultaneously with the secondary collision. However, since the driving rod 5 does not contract by its full length as it stands, but creates an impediment between the outer column 2 and the inner column 3 constituting the steering column 1, the steering column 1 does not contract. Needless to say, since the outer column 2 is supported relative to the vehicle such that it falls away forward simultaneously with the secondary collision due to the structure disclosed in Patent Documents 2 and 3 for example, even if there is a driving rod 5 as described above, it does not mean that there is no displacement of the steering wheel 8 forward at all. However, to the degree that the steering column 1 does not contract, not only does the peak of the shock load tend to occur at the initial stage of the secondary collision, but also the amount of forward displacement of the steering wheel 8 is reduced, and hence it is a disadvantage from the aspect of satisfactory protection of the driver.
Therefore, the abovementioned Patent Document 1 describes an invention related to a driving rod 5a that contracts by its full length based on a shock load applied during a secondary collision, using a structure as shown in FIG. 36 for example. The driving rod 5a comprises a cylindrical outer casing 10, and an internal shaft 11 joined by a pin 12 formed from a synthetic resin or a soft metal. The driving rod 5a as described above contracts by its full length as the pin 12 is broken by the shock load applied during the secondary collision. Accordingly, the driving rod 5a does not create an impediment between the outer column 2 and the inner column 3, and hence the steering column 1 is allowed to contract during the secondary collision.
However, it is difficult to ensure the concentricity of the driving rod 5a comprising the outer casing 10 and the internal shaft 11, over its full length. That is, the part toward one end and the part toward the other end of the driving rod 5a tend to be out of alignment. If it becomes eccentric, a non-uniform force acts on the part where the male thread 6 of the driving rod 5a and the nut screw together, and when rotating the driving rod 5a in order to adjust the fore-aft position of the steering wheel 8, annoying grinding noises, and uncomfortable vibrations are likely to occur.
Furthermore, it can also be considered that forming a nut, which constitutes a feed screw mechanism with a driving rod, from a synthetic resin, and breaking the female thread of the nut during the secondary collision, enables the displacement in the axial direction of the driving rod with respect to the nut, enabling the steering column to contract. However, in the case where such a structure is used, it is difficult to design to ensure the reliability (difficulty of breaking) of the threaded part of the male thread of the driving rod and the nut, and also to break the female thread reliably during the secondary collision.
[Patent Document 1] Japanese Patent Application Publication No. 2003-276616
[Patent Document 2] Japanese Patent Application Publication No. Hei 11-165643
[Patent Document 3] Japanese Patent Application Publication No. 2000-233758